The invention relates to the use of a biaxially oriented polypropylene film as an in-mould label during deep drawing.
The term “label films” covers a broad, technically complex field. A distinction is made between various labelling techniques, which differ fundamentally in terms of the process conditions involved, and necessarily impose different technical requirements on the label materials. Yet the common purpose of all labelling processes is produce attractive labelling for receptacles, one element of which is the labels must have good adhesion to the labelled receptacle.
A very wide range of techniques for applying the label are used in the labelling processes. There are self-adhesive labels, wrap-around labels, shrink labels, in-mould labels, and patch labels to name a few. It is entirely possible to use a film made from thermoplastic as the label in all of these various labelling processes.
Various techniques, in which different process conditions are applied, are also used in in-mould labelling. A feature shared by all in-mould labelling processes is that the label is included in the actual method by which the container is given its shape and during which it is applied. Here too, however, very different forming methods are used, for example injection moulding, blow moulding, deep drawing.
In the injection moulding method, a label is placed in the injection mould and is back-injected with a liquefied plastic. The label bonds with the injection moulded part under the high temperatures and pressures used and becomes an integral, inseparable part of the injection moulded item. This process is used for producing cups and lids for ice cream or margarine tubs, for example.
In this process, individual labels are taken from a stack or cut to size from a roll for placing in the injection mould. The mould is shaped so that stream of molten plastic is directed behind the label and front of the film is facing the wall of the injection mould. During injection, the hot molten plastic bonds with the label. After injection, the tool opens, the injection moulded item with the label is ejected and cools down. The label must adhere to the container without creases or any visible imperfections.
During moulding, the injection pressure is in a range from 300 to 600 bar. The plastics used have a melt flow index of about 40 g/10 min. The injection temperatures depend on the plastic that is being used. In some cases, the mould is also cooled to prevent the injection moulded item from sticking to the mould.
In-mould labelling may also be used in blow moulding of containers or hollow bodies. In this process, a fusion tube is extruded vertically downwards through an annular die. A vertically divided moulding tool closes around the tube, which is thus squeezed shut at the bottom end. At the top end, a blow mandrel is inserted and the opening of the moulded part is formed through this. The warm fusion tube is supplied with air through the blow mandrel, such that it expands until it is lying flush against the interior walls of the moulding tool. In this case, the label must bond with the highly viscous plastic of the fusion tube. Then, the mould is opened and the excess material on the moulded opening is cut off. The moulded, labelled container is ejected and cools down.
In these blow moulding processes, the pressure for expanding the fusion tube is approximately 4-15 bar and the temperatures significantly lower than in injection moulding. The plastic materials have a lower MFI than in injection moulding, to ensure that the fusion tube is dimensionally stable and they therefore behave differently during the cooling process from the low-viscosity materials used for injection moulding.
In deep drawing, thick, unorientated plastic panels, usually cast PP or PS (polystyrene), having a thickness of approximately 200-750 μm are heated and drawn or pressed into an appropriate moulding tool using a vacuum or punching tools. Here too, the individual label is placed in the mould and bonds with the actual container during the moulding process. This process makes use of substantially lower temperatures, so that adhesion of the label to the container may be a critical factor. Good adhesion must also be guaranteed at these low processing temperatures. The processing speeds in this method are lower than in with injection moulding.
In principle, films made of thermoplastics may also be used for labelling the containers during moulding in deep drawing. For this purpose, the films must have a selected property profile to ensure that the label film and the deep-drawn moulded body fit against one another smoothly without bubbles during the deep drawing and bond to one another.
The adhesion of the label to the container is frequently flawed, because comparatively lower temperatures and pressures are used in deep drawing than in injection moulding or blowmoulding methods. Furthermore, similarly to blowmoulding, air inclusions arise between the label and the container, which impair the appearance of the labelled container and also the adhesion. Therefore, special adhesion layers or highly specialised film structures have previously been proposed as labels for deep drawing applications.
A film of this type is described, for example, in WO 02/45956. The cover layer of this film has improved adhesive properties in respect of a great variety of materials. The cover layer contains a copolymer or terpolymer made of an olefin and unsaturated carboxylic acids or their esters as the main component. It is described that this film may also be used as a label in deep drawing because of the improved adhesion.
WO 2006/040057 describes the use of a biaxially oriented film with a microporous layer as a label in a deep drawing process. The microporous layer contains a propylene polymer and at least one β-nucleating agent. The microporosity is created by converting β-crystalline polypropylene when the film is stretched. Air inclusions are avoided due to the porous structure of the film. The adhesion of the film is surprisingly good. Therefore, the porous film may be used advantageously in deep drawing.
WO 98/32598 describes an in-mould label that comprises at least two layers, a sealable cover layer and a base layer. The sealing layer is applied to the surface of the base layer and contains a polyolefin having a melting temperature lower than 110° C. It is stated that 25% or less of this polyolefin melts at a temperature below 50° C. It is not explicitly stated that the film may also be used in deep drawing processes. The cover layer may be selected from a very wide range of different polymers.
EP 0 889 831 describes an in-mould layer comprising at least two layers. A first layer is a hot seal layer, which is stated to be activatable at a temperature of 57 to 100° C. The second layer contains vacuoles and a non-spherical, vacuole-initiating particle. According to the specification, the thermal conductivity of the film is less than 1.25*10-4 kcal/sec cm*° C. Only blowmoulding is cited as an example of an in-mould process. Deep drawing applications are not made explicitly evident in the document. The document also does not explain what is meant by the initiation temperature.
U.S. Pat. No. 6,764,760 describes in-mould labels comprising multilayer films that include a base layer, an intermediate layer and a sealable cover layer. Blowmoulding, injection moulding and deep drawing are cited as examples of in-mould processes. The sealable layer should be embossed to avoid blistering. The sealable layer is constructed from polyethylene having a melting point from 50-130° C. Modified polymers such as EVA or acrylate-modified polymers may optionally be included in the sealable cover layer as well.
An enormous variety of films that may generally be used as in-mould labels is described in the related art. However, the disclosed teachings frequently make no distinction between the various moulding processes, and thus invite the inference that the in-mould labels are able to be used interchangeably and equally well in the various processes. In the context of the present invention, it was found that this is usually not the case. For example, there are films that function extremely well in injection moulding, but cannot be used in blowmoulding due to excessive blistering and poor adhesion. Films that work well in blowmoulding have too little adhesion for use in deep drawing processes. In principle, each moulding process requires a special film whose properties are optimised for the precise conditions of the respective application. The usability of a film in a given application does not allow any conclusions to be drawn regarding the suitability of the material for use in another application.
In general, conventional wisdom among those skilled in the art holds that blistering and adhesion become progressively more problematic from injection moulding to blowmoulding and finally deep drawing, since the conditions for moulding the container become “more moderate” in this order. Deep drawing is carried out at the lowest temperatures and under the lowest pressures, with the result that the standard in-mould labels in deep drawing applications do not satisfy the requirements regarding label adhesion and freedom from blistering. For this reason, up to the present time in-mould labelling is not routinely carried out in deep drawing processes. In practice, deep drawn receptacles are either printed directly or provided with a wrap-around label.
The few known solutions that satisfy the requirements in technical terms involve sophisticated film production steps, and are therefore simply too expensive to be used instead of direct printing or wrap-around labelling. The acrylate-modified polymers described in. WO 02/45956 are so adhesive and sticky that the film has an undesirable tendency to stick to the rollers during production, and is often rendered completely unusable by excessive caking on the winding drum. In addition, if it can be processed far enough to form a label stack, the film is difficult to remove from the stack. Attempts to reduce the adhesive strength with admixtures or additives to a point that enables the film to be handled and unstacked during production, processing and application impair the adhesion so much that the film is no longer suitable for use in the deep drawing application, since adhesion to the container is no longer satisfactory.
The porous film described in WO 2006/040057 can only be manufactured at extremely slow production speeds, because the β-crystallites are only created in sufficient quantities if the extruded molten mass cools slowly. The film according to U.S. Pat. No. 6,764,760 only works for the deep drawing application with the suggested embossing, an extra processing step that also renders the film more expensive.
The known solutions that have been developed specifically for deep drawing are thus all based on extreme adhesive strength of the film surface or extreme surface roughness or special surface structures. The outer surfaces of these rough have comparatively low gloss values. The need for a film that is able to be used for in-mould labelling in deep drawing processes while meeting the requirements regarding adhesion, unstacking properties, freedom from blistering, gloss and integration in the container, and which is able to be manufactured in accordance with the standard, inexpensive film manufacturing processes has so far gone unanswered.
Moreover, it is desirable to provide a label whose advantageous glossy appearance is preserved after labelling as well. It often happens that the outer surface of films has an attractively lustrous appearance before processing, but after processing the structure of the film has been altered to such an extent that this gloss has been significantly reduced, or disappeared entirely, or has been completely destroyed by the “orange peel” effect. It is therefore the object of the present invention to provide an in-mould label that retains its bright visual appearance even after it has been applied to the container, that is to say the glossiness that the outer surface of the film has originally should be largely intact after the labelling process has been completed.